Video: The Sound of Raining

HW: TETRIS (7) & TETRIS (8)

Chapter 8 Destructor & Operator Overloading

Destructor A destructor is a function that is called when an object is no longer required. A constructor is a function which is called when a new object is created. A constructor is usually used to initiate an object. A destructor is usually used to destroy an object.

The Default Destructor The destructor for a class is a member function with the same name as the class, preceded by a tilde (~). For the CBox class, the prototype of the clas destructor is ~CBox(); A destructor has no parameters. Ex8_01.cpp on P.436 ~CBox() { cout << "Destructor called." << endl; } When some data members are dynamically allocated (see Chapter 4), a destructor must be used to destroy an object.

When Do We Dynamically Allocate Memory? Sometimes depending on the input data, you may allocate different amount of space for storing different types of variables at execution time int n = 0; cout << "Input the size of the vector - "; cin >> n; int vector[n]; error C2057: expected constant expression

Free Store (Heap, P.201) To hold a string entered by the user, there is no way you can know in advance how large this string could be. Free Store - When your program is executed, there is unused memory in your computer. You can dynamically allocate space within the free store for a new variable.

The new Operator Request memory for a double variable, and return the address of the space double* pvalue = NULL; pvalue = new double; Initialize a variable created by new pvalue = new double(9999.0); Use this pointer to reference the variable (indirection operator) *pvalue = 1234.0;

The delete Operator When you no longer need the (dynamically allocated) variable, you can free up the memory space. delete pvalue; Release memory pointed to by pvalue pvalue = 0; Reset the pointer to 0 After you release the space, the memory can be used to store a different variable later.

Allocating Memory Dynamically for Arrays Allocate a string of twenty characters char* pstr; pstr = new char[20]; delete [] pstr; Note the use of square brackets to indicate that you are deleting an array. pstr = 0; Set pointer to null

A Simple Example (cf. P.436) 3 a 5 b class CData { public: int* pdata; CData(int n=0) pdata = new(int); *pdata = n; cout << "Constructor called with initial value " << n << endl; } ~CData() cout << "Destructor called to release the memory storing " << *pdata << endl; delete pdata; }; int main() CData a(3); CData b(5); return 0; 3 a 5 b Constructor called with initial value 3 Constructor called with initial value 5 Destructor called to release the memory storing 5 Destructor called to release the memory storing 3

Q: Why Should I Write My Destructor? Why isn’t C++ compiler smart enough to automatically release the memory pointed by the pointer in my object? Consider some more complicated cases: 3 a 5 b c d

Class CMessage (1) P.438 Suppose you want to define a class Each object contains a text string. You don’t want to declare a data member as a large character array (like char [200]), So you’ll allocate memory in the free store for the message when an object is created. This is your constructor: CMessage(const char* text = "Default message") { pmessage = new char[strlen(text) + 1]; strcpy(pmessage, text); }

strlen, strcmp, strcpy #include <iostream> #include <cstring> using std::cout; using std::endl; int main() { char a[20] = "NCNU"; char b[20] = "Sunday"; cout << sizeof a << " " << strlen(a) << endl; // size = 20, string length = 4 if (strcmp(a,b) < 0) cout << "The string " << a << " is less than " << b << endl; strcpy(a, b); cout << a << endl; }

Destructors and Dynamic Memory Allocation (P.438) CMessage(const char* text = “Default message”) { pmessage = new char[strlen(text) + 1]; strcpy_s(pmessage, strlen(text)+1, text); } ~CMessage() cout << “Destructor called.” << endl; delete [] pmessage; A miss is as good as a mile. A cat can look at a queen. Destructor called. Q: What is the size of an object of the CMessage class?

Ex8_02.cpp on P.440 As the output indicates, the destructor is called only once. The object motto is created automatically, so the compiler also calls the destructor automatically. If you manually “delete pM”, it will free the memory pointed to by pM. Because the pointer pM points to a CMessage object, this causes the destructor to be invoked. Output becomes A miss is as good as a mile. A cat can look at a queen. Destructor called.

Dynamic Allocation Causes Troubles Not Only for Destructors class CData { public: int* pdata; CData(int n=0) pdata = new(int); *pdata = n; cout << "Constructor called with initial value " << n << endl; } void Print() cout << *pdata << endl; }; int main() CData a(3); CData b = a; a.Print(); b.Print(); *(a.pdata) = 5; 3 a 5 3 3 b 5 5

Behavior of a Default Copy Constructor CMessage motto1(“Radiation fades your genes.”); CMessage motto2(motto1); // Calls default copy constructor FIGURE 8-1 (P.442)

Q: What will the second motto2.ShowIt() display? CMessage motto1("A stitch in time saves nine."); CMessage motto2(motto1); motto2.ShowIt(); // Display 2nd message strcpy(motto1.pmessage, "Time and tide wait for no man."); motto1.ShowIt(); // Display 1st message Exercise: Let’s check figure8-1.cpp (P.442), but please ignore the error message at this moment. (Explained in next slides)

Implementing a Copy Constructor We don’t want the two objects sharing the same string in the memory. If motto1 is destroyed, the pointer in motto2 will become invalid. Let us implement a copy constructor to generate an object which is identical but independent of the old one. CMessage(const CMessage& initM) { pmessage = new char [ strlen(initM.pmessage) +1 ]; strcpy(pmessage, initM.pmessage); } Exercise: Modify Ex8_02.cpp (P.440) to implement this copy constructor.

Be sure to read the remaining part of Chapter 8 because we shall cover the topic of “operator overloading” in next class. Exercise Define a class CRational with two data members (numerator and denominator), and two member functions: Addition: Reduction (約分): Test your class with the following main program: int main() { CRational a(1, 4); CRational b(3, 4); CRational c = a.Addition(b); c.Print(); c.Reduction(); c.Print(); return 0; }